This invention relates to gate drivers for power MOSFETs, and more particularly, to a gate driver apparatus in which an energy recovering circuit is incorporated with a square-wave gate driver so that the turn-off loss in the gate driver is recovered.
Presently, gate drivers for power MOSFETs can be categorized as three major classes, i.e., the square-wave, quasi-resonant and resonant drivers. The square-wave gate driver as illustrated in FIG. 1 is typically implemented in an IC which consists of a pair of totem pole N and P-channel switches driven by the same control signal. The advantages of the square-wave driver are its simplicity in implementation and its independence of switching frequency. However, there is no scheme for recovering turn-on and turn-off energy losses resulting from charging and discharging of the gate capacitance of the MOSFETs, which can become intolerable for high frequency operation.
The quasi-resonant driver as shown in FIG. 2 can eliminate the turn-on loss in the gate driver, and the resonant driver as illustrated in FIG. 3 can eliminate both turn-on and turn off losses by circulating energy between the resonant inductor Lg and the effective gate capacitor Cg during each switching cycle. However, unlike the square-wave driver which can work in xe2x80x9cpulse width modulation (PWM)xe2x80x9d mode as well as in xe2x80x9cvariable frequency controlxe2x80x9d mode, both quasi-resonant and resonant drivers can work properly only in a narrowed frequency range of xe2x80x9cvariable frequency controlxe2x80x9d mode due to their dependence of switching frequency and duty cycle. Thus, the operating frequency of the circuit in the quasi-resonant and resonant drivers is severely limited.
Therefore, there exists a need of a gate driver which, on one hand, is switching frequency independent, and on the other hand, is capable of recovering some energy from the gate capacitance of the MOSFET.
The present invention provides a novel gate driver apparatus in which an energy recovery circuit is incorporated in a square-wave gate driver. The energy recovery circuitry is capable of recovering the turn-off loss by transferring the energy stored in the gate capacitor to the power source during a turn-off operation of the gate driver. In particular, the energy recovery circuitry comprises a first loop circuit for discharging the energy from the gate capacitor to an inductor when the gate driver is turned off, and a second loop circuit for discharging the energy from the inductor to the power supply. The first loop circuit is formed by turning on a first device, preferably a FET, and the second loop circuit is formed by utilizing an unidirectional device, preferably a diode. Thus, the energy of the gate capacitor is transferred to the power source when the gate driver is turned off, and the gate driver apparatus still maintains its operating flexibility as the square-wave driver which is independent of switching frequency.